1. Field of the Invention
The present invention relates to a secondary battery using a nonaqueous electrolyte solution (hereinafter referred to as a “nonaqueous secondary battery”), a method for making a negative electrode component used in the nonaqueous secondary battery, an apparatus for evaluating a graphite material for the negative electrode component, and an apparatus for making the graphite material.
2. Description of the Related Art
As rapid progress is made in the miniaturization of electronic devices, such as portable phones and notebook personal computers, secondary batteries are required to have higher energy densities.
In conventional secondary batteries, such as lead batteries, Ni—Cd batteries, and Ni-MH batteries, discharge voltages are low and energy densities are insufficient. Lithium secondary batteries are also used in practice, in which metallic lithium, lithium alloys, and carbonaceous materials which can electrochemically occlude and release lithium ions are used as negative electrode active materials, and various positive electrodes are used. The lithium secondary batteries have high output voltages and thus have large energy densities per weight or volume compared to the above conventional batteries.
In the lithium secondary batteries at initial stages, metallic lithium and lithium alloys are used as negative electrodes. A negative electrode using metallic lithium or a lithium alloy is insufficient in charge-discharge efficiency and has a problem in that dendritic lithium is formed. Thus, such negative electrodes are used in practice only in a few specialized fields.
Carbonaceous materials which can electrochemically occlude and release lithium ions have recently been anticipated as negative electrode components and are now coming into use. Negative electrodes using the carbonaceous materials do not have problems inherent in the metallic lithium or lithium alloys, that is, the formation of metallic lithium having a dendritic structure and particularization of the lithium alloy during charge-discharge cycles. Moreover, the carbonaceous materials show high coulomb efficiency; hence, lithium secondary batteries having carbonaceous negative electrodes exhibit superior charge-discharge reversibility.
In secondary batteries using the carbonaceous materials as negative electrode active materials, metallic lithium is not precipitated in use. Thus, lithium secondary batteries using the carbonaceous materials and nonflammable lithium compound oxide are safe and are commercially produced. These batteries are called “lithium ion batteries” and use carbonaceous materials as negative electrodes, LiCoO2 as positive electrodes, and nonaqueous electrolyte solutions containing nonaqueous solvents.
Carbonaceous materials used as negative electrodes are classified into graphite materials including natural products and artificial products, easily-graphitizable carbonaceous materials as precursors of artificial graphite materials, and ungraphitizable carbonaceous materials which are not converted to graphite even when these are treated at high temperatures facilitating the formation of graphite. Graphite materials and ungraphitizable carbonaceous materials have high capacities as negative electrodes and are thus currently used.
Lithium ion batteries have rapidly gained widespread use as electrical power sources in electronic devices, particularly, notebook personal computers due to compact, because they are lightweight, and have high capacities. Notebook personal computers having improved performance require higher CPU clock frequencies. Thus, high-performance computers consume significant amounts of electrical power and generate substantial amounts of heat during operation. Moreover, the restricted volume of dead space, which is inherent in miniaturization of personal computers, precludes the dissipation of heat which is generated during operation, resulting in an increase in the internal temperatures of personal computers.
The increased temperature accelerates deterioration and thus decreases capacity in batteries used in personal computers. The lost capacity cannot be restored by any means.